1. Technical Field to Which the Invention Pertains
This invention relates to an oscillator wherein a thermostatic oven is used to stabilize the oscillation frequency, and more particularly to a quartz-crystal oscillator wherein an operational amplifier is used to control a heat source of a thermostatic oven.
2. Background Art
From among oscillators which employ a quartz-crystal element, an oven-controlled oscillator wherein a vibrator such as a quartz-crystal element is held in a thermostatic oven has a stabilized oscillation frequency against a variation of the ambient temperature because an oscillation circuit operates with the quartz-crystal element or the like kept at a constant temperature. The oven-controlled oscillator is used particularly for applications for which a high frequency stability is required such as a comparatively high grade communication apparatus used in a base station of a mobile communication system, for example.
The oven-controlled oscillator usually includes a quartz-crystal element, a thermostatic oven which includes an electric heater and in which the quartz-crystal element is accommodated, and a heat source control circuit for controlling the heater of the thermostatic oven. FIG. 1 shows a general circuit configuration of a conventional oven-controlled oscillator.
The oven-controlled oscillator includes crystal oscillation circuit 10, thermostatic oven 1 in which quartz-crystal element 3 is accommodated, heater 6 for heating the inside of thermostatic oven 1, and heat source control circuit 2 for controlling heater 6. Heat source control circuit 2 includes thermistor RT1 thermally coupled to thermostatic oven 1 as hereinafter described, and the remaining part of heat source control circuit 2 except thermistor RT1 is referred to as control circuit unit 11. Crystal oscillation circuit 10 is an oscillation circuit that includes quartz-crystal element 3 as a circuit element. Crystal oscillation circuit 10 is a circuit of the Colpitts type, for example, wherein a resonance circuit is formed from quartz-crystal element 3 serving as an inductor component and series capacitors and part of an output of the resonance circuit is amplified and fed back to the resonance circuit by an amplifier (transistor) so that the circuit may oscillate.
As shown in FIG. 2, quartz-crystal element 3 is formed from a quartz blank of, for example, an AT cut enclosed in and held by metal vessel 5 from which a pair of leads 4 are led out. FIG. 3 illustrates a frequency-temperature characteristic of a quartz-crystal element which uses an AT cut quartz blank. As can be seen from FIG. 3, the AT cut quartz blank has such a frequency-temperature characteristic of a cubic curve that a point of inflection appears in the proximity of 25xc2x0 C. of the room temperature and a minimal value of the frequency appears in the proximity of +70xc2x0 C.
Thermostatic oven 1 is formed, as shown in FIG. 2 described above, from heater wire 6 serving as a heat source and wound around an outer wall of metal vessel 5 of quartz-crystal element 3. With thermostatic oven 1 having the configuration just described, metal vessel 5 is heated entirely by heater wire 6 and functions as a thermostatic oven for the quartz blank. According to circumstances, metal vessel 5 and heater wire 6 are insulated from the external air by means of a heat insulator or an adiabatic material (not shown). The internal temperature of thermostatic oven 1 is maintained constant by heat source control circuit 2. Heat source control circuit 2 detects the internal temperature of thermostatic oven 1 by means of a temperature sensitive element such as thermistor RT1 thermally coupled to thermostatic oven 1 and controls current to be supplied to heater 6 in response to a result of the detection to try to keep the internal temperature of thermostatic oven 1, that is, the temperature of quartz-crystal element 3, at a constant value. Thermistor RT1 is disposed in the proximity of heater wire 6, for example. The temperature of thermostatic oven 1 (quartz-crystal element 3) when such temperature control is performed is set to a temperature that indicates a frequency minimal value on the higher temperature side of the frequency-temperature characteristic of quartz-crystal element 3. This temperature is referred to as preset temperature.
FIG. 4 shows an example of particular circuit configuration of heat source control circuit 2. Heat source control circuit 2 includes operational amplifier 7 operating as an inverted differential amplifier. A reference voltage produced by dividing a voltage of power supply Vcc by dividing bias resistors R1, R2 is inputted to a non-inverted input terminal (+) of operational amplifier 7. A comparison voltage obtained from dividing bias resistors R3, R4 is inputted to an inverted input terminal (xe2x88x92) of operational amplifier 7 through resistor Ra. Here, thermistor RT1 serving as a temperature sensitive element is used as bias resistor R3 on power supply Vcc side. This thermistor RT1 has such a temperature-resistance characteristic that the resistance value decreases as the temperature rises as seen in FIG. 5. The output terminal and the inverted input terminal of operational amplifier 7 are connected to each other through feedback resistor Rb. In the present circuit, the amplification factor A of operational amplifier 7 is represented by Rb/Ra. The output of operational amplifier 7 is connected through resistor R5 to the base of transistor 8 whose emitter is grounded. Heater wire 6 of thermostatic oven 1 is connected between the collector of transistor 8 and power supply Vcc.
In the present circuit, various circuit constants are set so that the resistance value of thermistor RT1 at the room temperature of 25xc2x0 C. is higher than that of resistor R4 and a great difference voltage may appear between the reference voltage which depends upon resistors R1, R2 and the comparison voltage which depends upon resistor R4 and thermistor RT1. Consequently, when power supply is made available, the great difference voltage is amplified in accordance with the amplification factor A of operational amplifier 7 and inputted to the base of transistor 8. Accordingly, high collector current flows to transistor 8 and a great amount of heat is generated from heater 6. Therefore, when power supply is made available, the internal temperature of thermostatic oven 1, i.e., the temperature of quartz-crystal element 3, rises suddenly.
As the internal temperature of thermostatic oven 1 rises, the resistance value of thermistor RT1 drops, and consequently, the comparison voltage decreases until it indicates a value near to the reference voltage. At this time, the internal temperature of thermostatic oven 1 reaches +70xc2x0 C. corresponding to the minimal value of the frequency-temperature characteristic of the quartz-crystal element. Accordingly, the oscillation frequency varies from a frequency at the room temperature to another frequency at the minimal point based on the frequency-temperature characteristic and thereafter remains stably at the frequency at the minimal point. Usually, in order to cause the oscillation frequency to be stabilized rapidly after power supply is made available, the amplification factor A of operational amplifier 7 is set to a high value such as approximately 50 to 100, for example, so that the internal temperature of thermostatic oven 1 may rise rapidly.
In recent years, miniaturization of communication apparatus has been and is proceeding, and with the miniaturization, a quartz-crystal element of a reduced size is used popularly. Accordingly, also the heat capacity of the quartz-crystal element as thermostatic oven 1 decreases and the speed of the response of the internal temperature of the thermostatic oven to the power applied to heater 6 increases. Thus, when the internal temperature of the thermostatic oven approaches the preset temperature mentioned hereinabove after power supply is made available, a ringing phenomenon that the internal temperature of the thermostatic oven repeats a rise (overshoot) and a drop (undershoot) across the preset temperature occurs as shown in FIG. 6. Once a ringing phenomenon occurs, the time required for stabilization of the oscillation frequency increases, and this increases a substantial rise time. In an extreme case, a ringing phenomenon so continues as to cause a failure of equipment. This ringing phenomenon sometimes occurs not only when power supply is made available but also when the power supply increases or decreases due to a variation of the ambient temperature, for example, during operation because of an oversensitive reaction of the internal temperature of the oven, and this sometimes varies the oscillation frequency suddenly.
A possible solution to the problem described above is to set the amplification factor A of operational amplifier 7 to a lower value to decrease the power supply to heater 6 to prevent the ringing. However, the method of decreasing the amplification factor A gives rise to increase of the time required until the internal temperature of the thermostatic oven rises up to the preset temperature. Therefore, the method cannot be applied actually.
It is an object of the present invention to provide an oven-controlled oscillator wherein the internal temperature of a thermostatic oven rises rapidly up to a preset temperature while a ringing phenomenon is prevented and the oscillator exhibits a good rise after power supply is made available.
The object described above is achieved by a heat source control circuit (2) for controlling power to be supplied to a heater (6) serving as a heat source of a thermostatic oven (1), wherein a thermistor (RT2) is connected in series to a feedback resistor (Rb1) for an operational amplifier (7) and thermally coupled to the thermostatic oven (1).
In the present invention, the thermistor (RT2) whose resistance value varies in response to the internal temperature of the thermostatic oven is inserted in the feedback resistor for the operational amplifier. Therefore, when the internal temperature of the thermostatic oven is equal to or around the room temperature, the resistance value of the thermistor (RT2) is high and the amplification factor of the operational amplifier is high. On the other hand, in the proximity of the preset temperature on the high temperature side, the resistance value of the thermistor (RT2) is low and the amplification factor is low. Accordingly, in the proximity of the room temperature, high power is supplied to the heat source and the internal temperature of the thermostatic oven rises rapidly. Since the amplification factor decreases as the internal temperature of the thermostatic oven approaches the preset temperature, also the power supply to the heat source decreases, and consequently, the internal temperature of the thermostatic oven rises moderately up to the preset temperature. An excessive overshoot after the internal temperature of the thermostatic oven reaches the preset temperature is prevented. Furthermore, even if the ambient temperature varies after the preset temperature is reached, the heat source is prevented from reacting excessively sensitively and the internal temperature of the thermostatic oven is maintained constantly. In this manner, according to the present invention, an oscillator can be provided wherein the internal temperature of a thermostatic oven rises rapidly up to a preset temperature while a ringing phenomenon is prevented and the oscillator exhibits a good rise characteristic after power supply is made available and besides the oscillation frequency is stable.